Vehicles may be configured with transmissions wherein gears are automatically shifted responsive to vehicle operating conditions. During gear upshifts, engine speed is reduced. Then, following the upshift, the engine speed is returned to an increased level. To provide good driveability, following the gear upshift, power losses to the wheels (due to the engine speed reduction), may be compensated for by providing more engine torque. Vehicle engines may use boosting devices, such as turbochargers, to provide more aircharge and achieve increased engine torque. However, due to turbocharger inertia, the turbocharger speed, and consequently turbocharger boost, desired during and following the upshift is delayed with respect to the engine speed change. As such, this turbo lag results in the feeling of a poor and slow shift, and reduces vehicle performance.
Various control strategies may be used to adjust a turbocharger boost during transmission upshifts. One example approach is illustrated by Beaty in U.S. Pat. No. 6,692,406 B2. Herein, a selected engine operating parameter, such as a fuel injection setting or a wastegate control valve setting, is modulated to maintain the boost pressure during an upshift, and thereby reduce turbo lag. Specifically, in an engine operating in a maximum foot pedal position and an associated full-load condition, the selected engine operating parameter is modulated by maintaining the rotational speed of the turbocharger while the engine speed is reduced.
However, the inventors have recognized several potential issues with such a method. As one example, the approach involves maintaining the boost pressure at an elevated level during and following the gear upshift. As such, this approach may reduce turbo lag under maximum foot pedal position and engine full-load conditions. However, under conditions when the foot pedal position is not maximal, and engine loads are lower, maintaining boost pressure at the elevated level may substantially reduce fuel economy at the lower gear. As another example, the approach entails operating the wastegate valve with open loop control to maximize the turbine's power during the otherwise reduced airflow condition of the transmission upshift. However, as noted above, under conditions when the load is less than full and/or the pedal position is sub-maximal, operating in this way may substantially reduce fuel economy.
Thus in one example, some of the above issues may be addressed by a method for controlling a vehicle engine, the engine including a turbocharger and a transmission. One example method comprises, operating the transmission at a first lower gear with a first boost level, increasing the boost from the first boost level before completing a shift from the first lower gear to a second higher gear, and after completing the shift, operating the transmission at the second higher gear with the increased boost.
For example, based at least on a vehicle speed and engine load profile, a transmission gear upshift may be anticipated. Based on the prediction of an upshift, a drop in engine speed following the upshift may also be anticipated. To compensate for potential torque losses responsive to the drop in engine speed, an elevated boost may be scheduled following the gear upshift. Thus, in response to the upshift prediction, an engine controller may be configured to preposition the boost. That is, a boost build-up to the desired elevated level may be initiated responsive to the upshift prediction, for example, shortly after the boost prediction, but before the shift is completed. In one example, the boost level may be elevated by decreasing a degree of opening, and/or a duration of opening, of a turbocharger wastegate valve. The elevated boost may be blended in during a transition period preceding the upshift such that the elevated boost is attained, and held in reserve, substantially before the actual gear upshift. Torque disturbances arising during the transition period due to the elevated boost may be compensated using throttle adjustments. In this way, the elevated boost level needed immediately following a transmission upshift may be provided. In so doing, the quality of transmission shifts and engine performance may be improved. Furthermore, it may be possible to shift earlier during vehicle acceleration, thus further improving overall fuel efficiency of the powertrain, while also overcoming any turbocharger lag.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.